Filename: 143-distributed-storage-improvements.txt
Title: Improvements of Distributed Storage for Tor Hidden Service Descriptors
Author: Karsten Loesing
Created: 28-Jun-2008
Status: Superseded

Change history:

  28-Jun-2008  Initial proposal for or-dev

Overview:

  An evaluation of the distributed storage for Tor hidden service
  descriptors and subsequent discussions have brought up a few improvements
  to proposal 114. All improvements are backwards compatible to the
  implementation of proposal 114.

Design:

  1. Report Bad Directory Nodes

  Bad hidden service directory nodes could deny existence of previously
  stored descriptors. A bad directory node that does this with all stored
  descriptors causes harm to the distributed storage in general, but
  replication will cope with this problem in most cases. However, an
  adversary that attempts to make a specific hidden service unavailable by
  running relays that become responsible for all of a service's
  descriptors poses a more serious threat. The distributed storage needs to
  defend against this attack by detecting and removing bad directory nodes.

  As a countermeasure hidden services try to download their descriptors
  every hour at random times from the hidden service directories that are
  responsible for storing it. If a directory node replies with 404 (Not
  found), the hidden service reports the supposedly bad directory node to
  a random selection of half of the directory authorities (with version
  numbers equal to or higher than the first version that implements this
  proposal). The hidden service posts a complaint message using HTTP 'POST'
  to a URL "/tor/rendezvous/complain" with the following message format:

    "hidden-service-directory-complaint" identifier NL

      [At start, exactly once]

      The identifier of the hidden service directory node to be
      investigated.

    "rendezvous-service-descriptor" descriptor NL

      [At end, Excatly once]

      The hidden service descriptor that the supposedly bad directory node
      does not serve.

  The directory authority checks if the descriptor is valid and the hidden
  service directory responsible for storing it. It waits for a random time
  of up to 30 minutes before posting the descriptor to the hidden service
  directory. If the publication is acknowledged, the directory authority
  waits another random time of up to 30 minutes before attempting to
  request the descriptor that it has posted. If the directory node replies
  with 404 (Not found), it will be blacklisted for being a hidden service
  directory node for the next 48 hours.

  A blacklisted hidden service directory is assigned the new flag BadHSDir
  instead of the HSDir flag in the vote that a directory authority creates.
  In a consensus a relay is only assigned a HSDir flag if the majority of
  votes contains a HSDir flag and no more than one third of votes contains
  a BadHSDir flag. As a result, clients do not have to learn about the
  BadHSDir flag. A blacklisted directory node will simply not be assigned
  the HSDir flag in the consensus.

  In order to prevent an attacker from setting up new nodes as replacement
  for blacklisted directory nodes, all directory nodes in the same /24
  subnet are blacklisted, too. Furthermore, if two or more directory nodes
  are blacklisted in the same /16 subnet concurrently, all other directory
  nodes in that /16 subnet are blacklisted, too. Blacklisting holds for at
  most 48 hours.

  2. Publish Fewer Replicas

  The evaluation has shown that the probability of a directory node to
  serve a previously stored descriptor is 85.7% (more precisely, this is
  the 0.001-quantile of the empirical distribution with the rationale that
  it holds for 99.9% of all empirical cases). If descriptors are replicated
  to x directory nodes, the probability of at least one of the replicas to
  be available for clients is 1 - (1 - 85.7%) ^ x. In order to achieve an
  overall availability of 99.9%, x = 3.55 replicas need to be stored. From
  this follows that 4 replicas are sufficient, rather than the currently
  stored 6 replicas.

  Further, the current design stores 2 sets of descriptors on 3 directory
  nodes with consecutive identities. Originally, this was meant to
  facilitate replication between directory nodes, which has not been and
  will not be implemented (the selection criterion of 24 hours uptime does
  not make it necessary). As a result, storing descriptors on directory
  nodes with consecutive identities is not required. In fact it should be
  avoided to enable an attacker to create "black holes" in the identifier
  ring.

  Hidden services should store their descriptors on 4 non-consecutive
  directory nodes, and clients should request descriptors from these
  directory nodes only. For compatibility reasons, hidden services also
  store their descriptors on 2 consecutive directory nodes. Hence, 0.2.0.x
  clients will be able to retrieve 4 out of 6 descriptors, but will fail
  for the remaining 2 descriptors, which is sufficient for reliability. As
  soon as 0.2.0.x is deprecated, hidden services can stop publishing the
  additional 2 replicas.

  3. Change Default Value of Being Hidden Service Directory

  The requirements for becoming a hidden service directory node are an open
  directory port and an uptime of at least 24 hours. The evaluation has
  shown that there are 300 hidden service directory candidates in the mean,
  but only 6 of them are configured to act as hidden service directories.
  This is bad, because those 6 nodes need to serve a large share of all
  hidden service descriptors. Optimally, there should be hundreds of hidden
  service directories. Having a large number of 0.2.1.x directory nodes
  also has a positive effect on 0.2.0.x hidden services and clients.

  Therefore, the new default of HidServDirectoryV2 should be 1, so that a
  Tor relay that has an open directory port automatically accepts and
  serves v2 hidden service descriptors. A relay operator can still opt-out
  running a hidden service directory by changing HidServDirectoryV2 to 0.
  The additional bandwidth requirements for running a hidden service
  directory node in addition to being a directory cache are negligible.

  4. Make Descriptors Persistent on Directory Nodes

  Hidden service directories that are restarted by their operators or after
  a failure will not be selected as hidden service directories within the
  next 24 hours. However, some clients might still think that these nodes
  are responsible for certain descriptors, because they work on the basis
  of network consensuses that are up to three hours old. The directory
  nodes should be able to serve the previously received descriptors to
  these clients. Therefore, directory nodes make all received descriptors
  persistent and load previously received descriptors on startup.

  5. Store and Serve Descriptors Regardless of Responsibility

  Currently, directory nodes only accept descriptors for which they think
  they are responsible. This may lead to problems when a directory node
  uses an older or newer network consensus than hidden service or client
  or when a directory node has been restarted recently. In fact, there are
  no security issues in storing or serving descriptors for which a
  directory node thinks it is not responsible. To the contrary, doing so
  may improve reliability in border cases. As a result, a directory node
  does not pay attention to responsibilty when receiving a publication or
  fetch request, but stores or serves the requested descriptor. Likewise,
  the directory node does not remove descriptors when it thinks it is not
  responsible for them any more.

  6. Avoid Periodic Descriptor Re-Publication

  In the current implementation a hidden service re-publishes its
  descriptor either when its content changes or an hour elapses. However,
  the evaluation has shown that failures of hidden service directory nodes,
  i.e. of nodes that have not failed within the last 24 hours, are very
  rare. Together with making descriptors persistent on directory nodes,
  there is no necessity to re-publish descriptors hourly.

  The only two events leading to descriptor re-publication should be a
  change of the descriptor content and a new directory node becoming
  responsible for the descriptor. Hidden services should therefore consider
  re-publication every time they learn about a new network consensus
  instead of hourly.

  7. Discard Expired Descriptors

  The current implementation lets directory nodes keep a descriptor for two
  days before discarding it. However, with the v2 design, descriptors are
  only valid for at most one day. Directory nodes should determine the
  validity of stored descriptors and discard them one hour after they have
  expired (to compensate wrong clocks on clients).

  8. Shorten Client-Side Descriptor Fetch History

  When clients try to download a hidden service descriptor, they memorize
  fetch requests to directory nodes for up to 15 minutes. This allows them
  to request all replicas of a descriptor to avoid bad or failing directory
  nodes, but without querying the same directory node twice.

  The downside is that a client that has requested a descriptor without
  success, will not be able to find a hidden service that has been started
  during the following 15 minutes after the client's last request.

  This can be improved by shortening the fetch history to only 5 minutes.
  This time should be sufficient to complete requests for all replicas of a
  descriptor, but without ending in an infinite request loop.

Compatibility:

  All proposed improvements are compatible to the currently implemented
  design as described in proposal 114.

